Anaerobic digestion of bioethanol stillage for biogas energy production and nutrient and water recovery

Global bioethanol production has expanded in recent decades due to its potential as a renewable liquid vehicle fuel. Yet, its commercial growth is limited by narrow profit margins. Anaerobic digestion of stillage may improve the economics of bioethanol production by providing energy for process heating. Biogas and methane potential yields of 628 mL biogas/g VS and 373 mL CH4/g VS, respectively, were determined through batch experiments. Additionally, a laboratory-scale digester was fed with stillage for six months, with stable operation at organic loading rates up to 4 g VS/L/day and yields of 2.40 L biogas/L/day and 1.37 L CH4/L/day. Based on experimental data, a process model was developed using SuperPro Designer© to aid in engineering design and analysis. It was found that for a bioethanol plant processing 100 tons of sugar beets per day, anaerobic digestion of the stillage could supply enough methane to cover the majority of process heating. Solid-liquid separation of the digestate could contribute additional value for the potential use of the pressate for fertilizer and the solid matter as soil amendment. The model results suggest that anaerobic digestion could improve the economic viability of a sugar beet ethanol plant, and may offer similar improvements for other biofuel operations.